[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Quentin Colombet via llvm-commits]

https://github.com/qcolombet commented:

I feel like there are at least three different changes in this PR and they would need to be broken down:
1. The LICM changes
2. The RDA changes
3. The CSR changes for RISC-V

I think the overall direction makes sense. I'm not super happy how the CFI are handled but I haven't given it much thoughts.

Disclaimer, I haven't looked at the RISC-V changes at all.

https://github.com/llvm/llvm-project/pull/90819